Sensors are used in countless applications throughout a myriad of industries based upon their ability to respond in a measurable fashion to a stimulus. Sensors are used to measure or respond to everything from the presence of hazardous liquids or gases in a manufacturing process to the current weather conditions anywhere on the planet to stolen goods passing out the door of a retail establishment. With the advent of increasingly automated manufacturing processes as well as the possibility of hazardous environmental conditions caused by natural or induced disaster, gas sensors, such as may be utilized to discern the presence of gas or vapor-born pathogens, have attracted increasing attention of late.
In general, gas sensors are those sensors which convert a chemical signal to an electrical signal, with a change in the chemical input leading to a measurable change in the electrical output. Traditionally, gas sensors have been formed using semi-conducting oxides as the sensing material. These materials require expensive microfabrication techniques to form the sensors, however, and often only have the high sensitivities required to discern materials in low concentrations (such as parts per million) at high temperature (e.g., 200–600° C.).
Recently, carbon nanotubes have been examined as possible gas sensing materials due to their electrical and mechanical properties as well as their high specific surface area. For example, chemical sensors based on individual nanotubes have been reported by Kong, et al. (“Nanotube Molecular Wires as Chemical Sensors,” Science, 287, 622 (2000)). These sensors utilize the measurable change in the electrical resistance of a nanotube upon exposure to gases like NO2 and NH3 to sense the presence of the gas.
Inductor-capacitor (LC) resonant sensors are a type of solid-state sensor based upon the permittivity of a material. LC resonant sensors have been widely utilized as remote anti-theft sensors. In these remote sensing systems, an RF transmitter/receiver sends a microwave signal at a targeted frequency through an interrogation zone. The presence in the interrogation zone of an activated tag including a dielectric material having the targeted resonant frequency can be detected by the receiver, which then sets off an alarm.
More recently, research has been carried out to expand the use of LC resonant sensors beyond these simple yes- or no-type applications. Specifically, LC resonant sensors have been combined with carbon nanotube materials for utilization as gas sensors. For example, Ong, et al. (“A Wireless, Passive Carbon Nanotube-Based Gas Sensor,” IEEE Sensors Journal, Vol. 2, No. 2, April, 2002) has described a gas sensor formed of a responsive multi-wall carbon nanotube/silicon dioxide composite layer deposited on a planar LC resonant circuit. As the permittivity and/or conductivity of the MWNT/SiO2 composite changes with adsorption of CO2, O2, or NH3, so does the resonant frequency of the sensor, which can be remotely monitored through a loop antenna. The sensors showed reversible response to O2 and CO2, and an irreversible response to NH3.
Chopra, et al. (“Carbon-nanotube-based Resonant-circuit Sensor for Ammonia,” Applied Physics Letters, Volume 8, Number 24, 2002, which is incorporated herein in its entirety by reference thereto) have described an ammonia sensor formed of a simple micro-strip circular disk resonator coated with carbon nanotubes (either single-walled or multi-walled nanotubes) on the surface. The sensors show a shift in resonant frequency upon adsorption of ammonia of about 4.375 MHz for a single-walled nanotube (SWNT) sensor and a shift of about 3.25 MHz for a multi-walled nanotube (MWNT) sensor, and can detect the presence of ammonia down to a concentration of about 100 ppm.
Despite these advances in addressing the needs for improved gas sensors, there remains room for variation and improvement within the art.